In atomic energy electric generating plants, and similar facilities, there are many pipes carrying high pressure or high temperature fluids such as steam, water, sodium and other material. These pipes are supported and spaced from the walls or ceilings of a building or other surrounding support structure. In geographical areas where frequent earthquakes occur, there is serious concern that such pipes may rupture during an earthquake because of the rapid relative movement between the pipes and the rest of the building. Accordingly, there is a current requirement that snubbing devices be attached to the pipes and the adjacent support structure to prevent or dampen such rapid relative movement. During normal operation, the pipe must be able to expand and contract a considerable amount because of the changing temperature differentials, particularly during start up and shut down of the facility. Hence, the snubber must permit this slow relative movement while preventing the rapid movement of the pipe caused by an earthquake. After snubbing rapid movement, the device must recover automatically to once more permit slow movement of the pipe in either direction relative to its surrounding support structure. The snubber must of course be highly reliable for long periods of time because of the expense of providing access to repair or replace a malfunctioning snubber. Because of radiation and heat, maintenance presents a health hazard also. Moreover, the snubbing capability must remain ready at all times even though it may never be used. The failure of the snubber could of course result in a safety hazard and huge economic losses if a large power generating station should be shut down or destroyed.
The earliest snubbers were primarily hydraulic. However, a major shortcoming of hydraulic snubbers is that the hydraulic fluid and the seals employed deteriorate after a period of time, particularly when subjected to radiation. The hydraulic fluid tends to become a rubbery substance while the seals lose their flexibility and become hard and ceramic-like. Frequent servicing of hydraulic units is therefore necessary and is expensive.
The maintenance problems associated with hydraulic snubbers were significantly reduced by the development of a mechanical snubbing device described in "Acceleration Sensitive Motion Snubber", U.S. Pat. No. 3,876,040, assigned to the same assignee as the invention herein. The device does not rely upon hydraulic fluid, and thus is not affected by radiation or heat.
Although the aforementioned device significantly reduced the maintenance requirements, any mechanical device is subject to failures. Furthermore, a substantial number of hydraulic snubbers and other more failure-prone devices are still in wide use. Therefore, maintenance requirements still exist.
Because of the high heat and the potential radiation hazard, routine inspection and maintenance of the snubbers in an atomic energy electric generating plant is not feasible. Typically, a maintenance plan requires that 10% of the snubbers in a facility be inspected during the periodic refueling shut downs of the plant. Since the refueling shut downs may only occur every eighteen months or so, each snubber is inspected only once every fifteen years under this plan. It is not at all unlikely that the inspected snubbers will be in good working order while a defective snubber can remain undetected for a number of years.
Since the obvious alternative of sending a larger crew in to inspect all snubbers during each maintenance cycle is not economically feasible, nor is it desirable from a health hazard standpoint, an alternative means for detecting defective snubbers is desirable. Thus far, no such means have been available.